Semiconductor devices

ABSTRACT

Semiconductor device is provided. The semiconductor device includes a base substrate including a first device region, a second device region, and a transition region separating the first region from the second region. A first work function layer is formed on the base substrate in the second region. A second work function layer is formed on the base substrate in the first region and the transition region, and on the first work function layer in the second region.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 15/985,278, filed on May 21, 2018, which claims the priority of Chinese Patent Application No. 201710364138.3, filed on May 22, 2017, the entire content of which is incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to the field of semiconductor fabrication technology and, more particularly, relates to semiconductor devices.

BACKGROUND

With the rapid development of semiconductor technology, the feature size of semiconductor structures is continuously reduced, and the integration level of integrated circuits (ICs) is steadily improved. Accordingly, the requirements on device performance are also becoming higher and higher.

Currently, with the dimension of metal-oxide-semiconductor field-effect transistor (MOSFET) continuously becoming smaller, in order to accommodate the reduction of the process node, the channel length in MOSFET devices may have to be reduced, which may be conducive to increasing the density of the chip, improving the switching speed of the MOSFET devices, etc.

However, as the channel length of devices decreases, the distance between the source region and the drain region may also be reduced. Therefore, the ability of the gate structure in controlling the channel may be degraded such that pinching off the channel by the gate may be more and more difficult. As such, the sub-threshold leakage phenomenon, i.e. the short-channel effect (SCE), becomes a crucial technical challenge and needs to be resolved.

In order to accommodate the requirements for scaling-down semiconductor devices, semiconductor process gradually switches from planar MOSFET devices to more efficient three-dimensional (3D) transistor devices, such as fin field-effect transistors (Fin-FETs). Fin-FET devices demonstrate desired ability in control the channels.

However, the electrical performance of the conventional semiconductor structures may still need to be improved. The disclosed semiconductor devices and fabrication methods thereof are directed to solve one or more problems set forth above and other problems in the art.

BRIEF SUMMARY OF THE DISCLOSURE

One aspect of the present disclosure provides a method for fabricating a semiconductor device. The method includes providing a base substrate including a first device region, a second device region, and a transition region separating the first region from the second region; forming a first work function layer on the first region, the transition region, and the second region of the base substrate; removing a first portion of the first work function layer formed in the transition region; forming a hard mask layer on the base substrate in the transition region and on the first work function layer in the second region; removing a second portion the first work function layer formed in the first region using the hard mask layer as an etch mask; removing the hard mask layer; and forming a second work function layer, on the base substrate in the first region and the transition region, and on the first work function layer in the second region.

Another aspect of the present disclosure provides a semiconductor device. The semiconductor device includes a base substrate including a first device region, a second device region, and a transition region separating the first region from the second region; a first work function layer formed on the base substrate in the second region; and a second work function layer formed on the base substrate in the first region and the transition region, and on the first work function layer in the second region.

Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are merely examples for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present disclosure.

FIGS. 1-5 illustrate schematic views of semiconductor devices at certain stages of a method for fabricating a semiconductor device;

FIGS. 6-13 illustrate schematic views of semiconductor devices at certain stages of an exemplary fabrication method for a semiconductor device consistent with various embodiments of the present disclosure; and

FIG. 14 illustrates a flowchart of the exemplary method for fabricating the semiconductor device consistent with various embodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIGS. 1-5 illustrate schematic views of semiconductor devices at certain stages of a method for fabricating a semiconductor device.

Referring to FIG. 1 , a substrate 100 is provided. A plurality of discrete fin structures 110 are formed on the substrate 100. The substrate 100 includes a first region I used to form a plurality of first devices, and a second region II used to form a plurality of second devices. An isolation layer 120 is formed on the portion of the substrate 100 exposed by the plurality of fin structures 110. An interfacial dielectric layer 122 may be formed to cover the portion of each fin structure 110 formed above the isolation layer 120. Moreover, a high-k dielectric layer 121 is formed on the interfacial dielectric layer 122 and the portion of the isolation layer 120 exposed by the plurality of fin structures 110.

Referring to FIG. 2 , a first work function layer 130 is formed on the high-k dielectric layer 121.

Referring to FIG. 3 , a hard mask layer 140 is formed in the second region II to cover the plurality of fin structures 110 and the isolation layer 120.

Referring to FIG. 4 , the portion of the first work function layer 130 formed in the first region I is removed by etching using the hard mask layer 140 as an etch mask.

Referring to FIG. 5 , the hard mask layer 140 is removed, and a second work function layer 150 is formed on the high-k dielectric layer 121 in the first region and also on the first work function layer 130 in the second region II.

The electrical performance of the formed semiconductor device may not be desired due to several reasons. According to the fabrication process described above, after forming the hard mask layer 140 in the second region II on the fin structures 110 and the isolation layer 120, the portion of the first work function layer 130 formed in the first region I is then removed by etching using the hard mask layer 140 as the etch mask. Usually, a wet etching process is performed to remove the portion of the first work function layer 130 formed in the first region I. Although the hard mask layer is formed on the first work function layer 130 in the second region II, i.e., the first work function layer 130 in the second region II is located between the hard mask layer 140 and the high-k dielectric layer 121, the side surface of the first work function layer 130 (indicated by an arrow ‘A’ in FIG. 4 ) may still be exposed to the etch solution during the wet etching process. Therefore, the etch solution may also be able to etch the portion of the first work function layer 130 formed in the second region II. As such, the quality of the first work function layer 130 in the second region II may be undesirably degraded. In addition, the etching effect of the etch solution may also lead to more surface lattice defects in the portion of the high-k dielectric layer 121 formed between the first region I and the second region II. As such, the quality of the subsequently-formed second work function layer 150 in the first region I may be undesirably degraded, and the electrical performance of the semiconductor device may need to be improved.

The present disclosure provides semiconductor devices with improved electrical performance and their fabricating methods. FIG. 14 illustrates a flowchart of the exemplary method for fabricating the semiconductor device consistent with various embodiments of the present disclosure. FIGS. 6-13 illustrate schematic views of semiconductor devices at certain stages of the exemplary fabrication method.

Referring to FIG. 14 , a base substrate may be provided, the base substrate including a substrate, a plurality of discrete fin structures formed on the substrate, an isolation structure formed on the substrate exposed by the fin structures, an interfacial dielectric layer formed on the fin structures, and a high-k dielectric layer formed on the interfacial dielectric layer and the isolation structure exposed by the fin structures, and the substrate including a first region used to form a plurality of first devices, a second region used to form a plurality of second devices, and a transition region separating the first region from the second region (S401). FIG. 6 illustrates a schematic cross-section view of a semiconductor device consistent with various embodiments of the present disclosure.

Referring to FIG. 6 , a base substrate may be provided. The base substrate may include a substrate 200 and a plurality of discrete fin structures 210 formed on the substrate 200. The substrate 200 may include a first region I used to form a plurality of first devices, a second region II used to form a plurality of second devices, and a transition region III separating the first region I from the second region II. In one embodiment, the dimension of the transition region III may be defined in a subsequent process.

The base substrate may also include an isolation structure 220 formed on the substrate 200 exposed by the fin structures 210, an interfacial dielectric layer 222 formed on the plurality of fin structures 220, and a high-k dielectric layer 221 formed on the interfacial layer 222 and also on the portion of the isolation structure 220 exposed by the fin structures 210.

In one embodiment, the semiconductor device to be formed is a fin field-effect transistor (Fin-FET) device. In other embodiments, the semiconductor device to be formed may have a planar structure, and accordingly, the base substrate may be a planar substrate.

The type of the plurality of first devices formed in the first region I may be different from the type of the plurality of second devices formed in the second region II. In one embodiment, the plurality of first devices are N-type metal-oxide-semiconductor (NMOS) devices and the plurality of second devices are P-type metal-oxide-semiconductor (PMOS) devices. In other embodiments, the plurality of first devices may be PMOS devices and the plurality of second devices may be NMOS devices.

In one embodiment, the substrate 200 may be made of Si. In other embodiments, the substrate may be made of Ge, SiGe, SiC, GaAs, InAs, or any other appropriate semiconductor material. Alternatively, the substrate may be made of Si on insulator (SOI), Ge on insulator (GOI), or any other appropriate composite structure.

In one embodiment, the plurality of fin structures 210 may be made of Si. In other embodiments, the plurality of fin structures may be made of Ge, SiGe, SiC, GaAs, InAs, or any other appropriate semiconductor material.

In one embodiment, the isolation structure 220 may be used to electrically isolate neighboring fin structures 210. The isolation structure 220 may be made of SiO₂. In other embodiments, the isolation structure may be made of SiN, SiON, or any other appropriate material.

In one embodiment, the interfacial dielectric layer 222 may be made of at least one of SiO₂ and SiON. The interfacial dielectric layer 222 may be formed by an oxidation process.

In one embodiment, the high-k dielectric layer 221 may be made of a high-k dielectric material. For example, the high-k dielectric material may include one or more of HfO₂, TaO₂, HfLaO, etc.

Referring back to FIG. 14 , a first work function layer may be formed on the high-k dielectric layer (S402). FIG. 7 illustrates a schematic cross-section view of a semiconductor structure consistent with some embodiments of the present disclosure.

Referring to FIG. 7 , a first work function layer 230 may be formed on the high-k dielectric layer 221. In one embodiment, the first work function layer 230 may be used to adjust the threshold voltages of subsequently-formed second devices. The first work function layer 230 may be made of one or more of TiN, TaN, TiSiN, and any other appropriate material.

In one embodiment, the thickness of the first work function layer 230 may be determined according to the requirements of the second devices. The thickness of the first work function layer 230 may not be too large or too small. When the thickness of the first work function layer 230 is too large or too small, the first work function layer 230 may not electrically match with the second devices. Therefore, in one embodiment, the thickness of the first work function layer 230 may be in a range of approximately 25 Å to 50 Å.

In one embodiment, the first work function layer 230 may be formed by an atomic layer deposition (ALD) process or a chemical vapor deposition (CVD) process.

Referring back to FIG. 14 , a protection layer may be formed on the first work function layer (S403). FIG. 8 illustrates a schematic cross-section view of a semiconductor structure consistent with some embodiments of the present disclosure.

Referring to FIG. 8 , a protection layer 240 may be formed on the first work function layer 230. In one embodiment, the protection layer may be used to isolate the first work function layer 230 from a subsequently-formed patterned layer. The protection layer 240 may be made of at least one of polycrystalline Si and amorphous Si.

Further, returning to FIG. 14 , a patterned layer may be formed on the protection layer, and the portion of the protection layer corresponding to the transition region may be exposed by the patterned layer (S404). FIG. 9 illustrates a schematic cross-section view of a semiconductor device consistent with some embodiments of the present disclosure.

Referring to FIG. 9 , a patterned layer 250 may be formed on the protection layer 240. The patterned layer 250 may expose a portion of the protection layer 240. Moreover, the exposed portion of the protection layer 240 may correspond to the transition region III.

In one embodiment, the patterned layer 250 may be used as an etch mask during a subsequently-performed etching process to remove the portion of each of the protection layer 240, the first work function layer 230, and the high-k dielectric layer 221 corresponding to the transition region III. The removed portion of the protection layer 240, the first work function layer 230, and the high-k dielectric layer 221 may correspond to the transition region III. That is, the patterned layer 250 may be used to define the dimension of the transition region III. In one embodiment, the patterned layer 250 may be made of a photoresist material.

Moreover, the dimension of the transition region III defined by the patterned layer 250 may not be too large or too small. That is, along a direction perpendicular to the length direction of the fin structures 210, the width of the transition region III may not be too large or too small. When the width of the transition region III is overly large, the integration level of the semiconductor device may be reduced. When the width of the transition region III is too small, the first region I and the second region II may not be effectively isolated from each other. As such, in a subsequent process, the removal of the portion of the first work function layer 230 formed in the first region I may have undesired effect on the portion of the first work function layer 230 formed in the second region II. Therefore, along the direction perpendicular to the length direction of the fin structures 210, the width of the transition region III may be in a range of approximately 36 nm to 86 nm.

Referring back to FIG. 14 , the portion of the protection layer exposed by the patterned layer together with the portion of the first work function layer and the high-k dielectric layer formed under the portion of the protection layer may be removed through an etching process using the patterned layer as an etch mask (S405). The schematic cross-section view of the semiconductor device in FIG. 9 illustrates the removal of the portion of the protection layer, the first work function layer, and the high-k dielectric layer.

Referring to FIG. 9 , after forming the patterned layer 250, by further using the patterned layer 250 as an etch mask, the portion of the protection layer 240 exposed by the patterned layer 250 together with a portion of the first work function layer 230 and the high-k dielectric layer 221 formed under the exposed portion of the protection layer 240 may be removed. That is, the portion of each of the protection layer 240, the first work function layer 230, and the high-k dielectric layer 221 formed in the transition region III may be removed.

Referring back to FIG. 14 , the patterned layer and the protection layer may both be removed (S406). FIG. 10 illustrates a schematic cross-section view of a semiconductor structure consistent with some embodiments of the present disclosure.

Referring to FIG. 10 , the patterned layer 250 and the protection layer 240 may be removed. In one embodiment, the protection layer 240 may be used to isolate the first work function layer 230 and the patterned layer 250. Moreover, the patterned layer 250 is usually removed by an ashing process. However, during the ashing process, the O, N, H, and other atoms in the patterned layer 250 may react with the first work function layer 230 by chemical reactions. Therefore, the presence of the protection layer 240 may be able to prevent the first work function layer 230 from contacting the patterned layer 250. As such, undesired chemical reaction between the first work function layer 230 and the patterned layer 250 may be avoided, and thus the quality of the first work function layer 230 may be improved.

In one embodiment, the process to remove the portion of the protection layer 240, the first work function layer 230, and the high-k dielectric layer 221 formed in the transition region III may be a dry etching process. For example, the process parameters adopted in the dry etching process may include an etch gas including CF₄, SF₆, Cl₂, and O₂, a flowrate of CF₄ in a range of approximately 10 sccm to 500 sccm, a flowrate of SF₆ in a range of approximately 20 sccm to 300 sccm, a flowrate of Cl₂ in a range of approximately 6 sccm to 120 sccm, a flowrate of O₂ in a rage of approximately 1 sccm to 90 sccm, a process pressure in a range of approximately 1 mTorr to 250 mTorr, and a process power in a range of approximately 100 W to 500 W.

In one embodiment, the process to remove the protection layer 240 may be different from the process to remove the patterned layer 250. That is, the patterned layer 250 may be removed before removing the protection layer 240. Moreover, the patterned layer 250 may be removed by an ashing process, and the protection layer 240 may be removed by a wet etching process or a dry etching process.

Referring back to FIG. 14 , a hard mask layer may be formed on the isolation structure in the transition region and also on the first work function layer in the second region (S407). FIG. 11 illustrates a schematic cross-section view of a semiconductor device consistent with some embodiments of the present disclosure.

Referring to FIG. 11 , a hard mask layer 260 may be formed on the isolation structure 220 in the transition region III and also on the first work function layer 230 in the second region II.

In one embodiment, the hard mask layer 260 may be used to protect the isolation structure in the transition region III and the first work function layer 230 in the second region II. The hard mask layer 260 may be made of one or more of SiN, SiCN, SiBN, SiOCN, and SiON.

Referring back to FIG. 14 , the portion of the first work function layer formed in the first region may be removed through an etching process using the hard mask layer as an etch mask (S408). FIG. 12 illustrates a schematic cross-section view of a semiconductor device consistent with some embodiments of the present disclosure.

Referring to FIG. 12 , the portion of the first work function layer 230 formed in the first region I may be removed by etching using the hard mask layer 260 as an etch mask.

According to the disclosed fabrication method, the portion of the protection layer 240 (referring to FIG. 9 ), the first work function layer 230, and the high-k dielectric layer 221 formed in the transition region III is removed by etching using the patterned layer 250 (referring to FIG. 9 ) as the etch mask such that the first region I and the second region II are separated or otherwise disconnected from each other. In addition, by using the hard mask layer 260 to protect the second region II and the transition region III, the process to remove the portion of the first work function layer 230 formed in the first region I may not easily affect the transition region III and the portion of the first work function layer 230 in the second region II. As such, the quality of the first work function layer 230 and the quality of a subsequently-formed second work function layer may both be improved. Accordingly, the second work function layer may be able to effectively adjust the threshold voltages of the first devices and the first work function layer 230 may be able to effectively adjust the threshold voltages of the second devices. Therefore, the electrical performance of the semiconductor device may be improved.

Moreover, according to the disclosed fabrication method, the process to remove the portion of the first work function layer 230 in the first region I using the hard mask layer 260 is a wet etching process. Specifically, because the first region I and the second region II are separated from each other by the transition region III, the etch solution used in the wet etching process may not likely etch the portion of the first work function layer 230 formed in the second region II. Therefore, the quality of the first work function layer 230 may be improved. In a subsequent process, a second work function layer may be formed on the portion of the high-k dielectric layer 221 in the first region I, the portion of the isolation structure 220 in the third region III, and the portion of the first work function layer 230 in the second region II. Because the quality of the first work function layer 230 may be improved, the quality of the second work function layer may also be improved.

Referring back to FIG. 14 , the hard mask layer may be removed, and then a second work function layer may be formed on the portion of the high-k dielectric layer in the first region, the portion of the isolation structure in the third region, and the portion of the first work function layer in the second region (S409). FIG. 13 illustrates a schematic cross-section view of a semiconductor device consistent with some embodiments of the present disclosure.

Referring to FIG. 13 , the hard mask layer 260 may be removed. Then, a second work function layer 270 may be formed on the portion of the high-k dielectric layer 221 in the first region I, the portion of the isolation structure 220 in the third region III, and the portion of the first work function layer 230 in the second region II.

In one embodiment, the second work function layer 270 may be used to adjust the threshold voltages for the plurality of first devices to be formed in the first region I. The second work function layer 270 may be made of one or more of TiAl, Al, TiC, and TiCAl. Moreover, the second work function layer 270 may be formed by an ALD process or a CVD process.

In one embodiment, the thickness of the second work function layer 270 may be determined based on the requirements of the first devices. The thickness of the second work function layer 270 may not be too large or too small. When the thickness of the second work function layer 270 is too large or too small, the second work function layer 270 may not electrically match with the first devices. Therefore, in one embodiment, the thickness of the second work function layer 270 may be in a range of approximately 30 Å to 60 Å.

According to the disclosed fabrication method, the transition region III is formed to separate the first region I and the second region II. Therefore, the quality of the first work function layer 230 and the quality of the second work function layer 270 may be desired. Accordingly, the first work function layer 230 may be able to effectively adjust the threshold voltages of the second devices and the second work function layer 270 may be able to effectively adjust the threshold voltages of the first devices. As such, the electrical performance of the semiconductor device may be improved.

Further, the present disclosure also provides a semiconductor device. FIG. 13 illustrates a schematic cross-section view of an exemplary semiconductor device consistent with some embodiments of the present disclosure.

Referring to FIG. 13 , the semiconductor device may include a base substrate. The base substrate may further include a substrate 200 and a plurality of discrete fin structures 210 formed on the substrate 200. The substrate 200 may include a first device region I used to form a plurality of first devices, a second device region II used to form a plurality of second devices, and a transition region III separating the first device region I and the second device region II.

The semiconductor device may include an isolation structure 220 formed on the portion of the substrate 200 exposed by the plurality of fin structures 210, an interfacial dielectric layer 222 formed on the plurality of fin structures 210, a high-k dielectric layer 221 formed on the portion of the isolation structure 220 exposed by the fin structures 210 in the first device region I and the second device region II and also on the interfacial dielectric layer 222, a first work function layer 230 formed on the portion of the high-k dielectric layer 221 in the second region II, and a second work function layer 270 formed on the portion of the high-k dielectric layer 221 in the first region I, the portion of the isolation structure in the transition region III, and the first work function layer 230 in the second region II.

In one embodiment, the type of the plurality of first devices formed in the first region I may be different from the type of the plurality of second devices formed in the second region II.

According to the disclosed semiconductor device, the transition region III separates the first region I and the second region II. Therefore, the quality of the first work function layer 230 and the quality of the second work function layer 270 may be desired. Accordingly, the first work function layer 230 may be able to effectively adjust the threshold voltages of the second devices and the second work function layer 270 may be able to effectively adjust the threshold voltages of the first devices. As such, the electrical performance of the semiconductor device may be improved.

Compared to conventional semiconductor devices and fabrication methods, the disclosed semiconductor devices and fabrication methods may demonstrate several advantages.

According to the disclosed semiconductor devices and fabrication methods, a portion of the first work function layer may be removed through an etching process such that a transition region may be defined to separate the first region and the second region. That is, a transition region may be defined to separate the plurality of first devices from the plurality of second devices. Further, a hard mask layer may be formed to cover the base substrate in the transition region and the portion of the first work function layer in the second region. In a subsequent process, the portion of the first work function layer formed in the first region may be removed. Because the first region and the second region are separated from each other, and the hard mask layer provides protection for the first work function formed in the second region and the base substrate in the transition region, the process to remove the portion of the first work function layer formed in the first region may not cause undesired effect on the transition region and the second region. Specifically, in the field of semiconductor manufacturing, a wet etching process is usually used to remove the portion of the first work function layer formed in the first region. Because the first region is separated from the second region by the transition region, the etch solution used in the wet etching process may not be likely etch the portion of the first work function layer formed in the second region, and thus the quality of the first work function layer may be improved. Further, a second work function layer is then formed on the base substrate and the first work function layer. Because of the improved quality of the first work function layer, the quality of the subsequently-formed second work function layer may also be improved. As such, the first work function layer may be able to effectively adjust the threshold voltages of the second devices formed in the second region and the second work function layer may be able to effectively adjust the threshold voltages of the first devices formed in the first region. Therefore, the electrical performance of the semiconductor device may be improved.

Optionally, during the etching process to remove the portion of the first work function layer corresponding to the transition region, the protection layer may be used to protect the portion of the first work function layer in the first region and the second region. Specifically, because the protection layer is formed between the first work function layer and the patterned layer, the protection layer is able to separate the first work function layer and the patterned layer. In a subsequent process, an ashing process is usually used to remove the patterned layer. Because during the ashing process, the O, N, H, and other atoms in the patterned layer may react with the first work function layer by chemical reactions. Therefore, the presence of the protection layer may be able to prevent the first work function layer from contacting the patterned layer. As such, undesired chemical reaction between the first work function layer and the patterned layer may be avoided, and thus the quality of the first work function layer may be improved.

The above detailed descriptions only illustrate certain exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention. Those skilled in the art can understand the specification as whole and technical features in the various embodiments can be combined into other embodiments understandable to those persons of ordinary skill in the art. Any equivalent or modification thereof, without departing from the spirit and principle of the present invention, falls within the true scope of the present invention. 

What is claimed is:
 1. A semiconductor device, comprising: a base substrate including a first region, a second region, and a transition region located between the first region and the second region; a first fin structure over the substrate in the first region; a second fin structure adjacent to the first fin structure and over the substrate in the second region; an isolation structure formed over the substrate between the first and second fin structures, the isolation structure having a top lower than the first and second fin structures; a high-k dielectric layer formed over and between the first and second fin structures, wherein the high-k dielectric layer includes an opening exposing a portion of the isolation structure in the transition region; a first work function layer formed over the high-k dielectric layer in the second region; and a second work function layer continuously extending along and on surfaces including: the high-k dielectric layer over the first fin structure in the first region, the exposed portion of the isolation structure in the transition region, and the first work function layer over the second fin structure in the second region.
 2. The semiconductor device according to claim 1, wherein: a width of the transition region is in a range of approximately 36 nm to 86 nm.
 3. The semiconductor device according to claim 1, wherein: a plurality of first devices are formed in the first region; and a plurality of second devices are formed in the second region.
 4. The semiconductor device according to claim 3, wherein: a type of the plurality of first devices formed in the first region is different from a type of the plurality of second devices formed in the second region.
 5. The semiconductor device according to claim 4, wherein: the plurality of first devices are N-type metal-oxide-semiconductor (NMOS) devices; and the plurality of second devices are P-type metal-oxide-semiconductor (PMOS) devices.
 6. The semiconductor device according to claim 5, wherein: the first work function layer is made of at least one of TiN, TaN, and TiSiN.
 7. The semiconductor device according to claim 1, wherein: a thickness of the first work function layer is in a range of approximately 25 Å and 50 Å.
 8. The semiconductor device according to claim 5, wherein: the second work function layer is made of at least one of TiAl, Al, TiC, and TiCAl.
 9. The semiconductor device according to claim 1, wherein: a thickness of the second work function layer is in a range of approximately 30 Å and 60 Å.
 10. The semiconductor device according to claim 1, further comprising: an interfacial dielectric layer enveloping each of the first and second fin structures over the isolation structure.
 11. The semiconductor device according to claim 1, wherein the isolation structure is made of at least one of SiO₂, SiN, or SiON. 